Difference between revisions of "Bash operators"

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You have a shell script and wonder what these operators do? Then this article is for you.
 +
 
= * and ? =
 
= * and ? =
 
 
You may already know these used for so called "shell globbing" (pattern matching). * replaces an arbitrary amount of characters (including none), while ? replaces exactly one character.
 
You may already know these used for so called "shell globbing" (pattern matching). * replaces an arbitrary amount of characters (including none), while ? replaces exactly one character.
  
 
These wildcards are actually bash operators! Imagine, we have the files a1, a2:
 
These wildcards are actually bash operators! Imagine, we have the files a1, a2:
 
  cp a*
 
  cp a*
 +
 
now is a valid bash command which overrides one of these files with the other one.
 
now is a valid bash command which overrides one of these files with the other one.
  
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The operator $() in the bash shell is replaced by the output of the command enclosed in the parentheses. It is equivalent to backticks (``), but can be cascaded more easily.
 
The operator $() in the bash shell is replaced by the output of the command enclosed in the parentheses. It is equivalent to backticks (``), but can be cascaded more easily.
  
Examples:
+
Examples
 
  rpm -ql $(rpm -qa)
 
  rpm -ql $(rpm -qa)
  
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  # echo $((2*2))
 
  # echo $((2*2))
 
  4
 
  4
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 +
= $$ =
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The operator $$ delivers the id of the currently-running process.
 +
 +
Example:
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# kill -9 $$
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 +
kills the current process
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= $! =
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The operator $! delivers the id of the process that has most recently sent to the [[background]].
 +
 +
Example:
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# [[xosview]] & [[sleep]] 5; [[kill]] $!
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 +
shows xosview for 5 seconds and stops it
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 +
= > and >> =
 +
 +
> and >> can be used to redirect the output (only stdout, not stderr, for that see below) of a command to a file. The difference between them is, that >> appends to the given files, while > will truncate it.
 +
 +
Examples:
 +
ls -alh > files-in-directory.txt
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ls -alh /otherdir >> files-in-directory.txt  # Append list of files in other directory
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 +
= 2> or 2>> =
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[[stderr]] can be redirected by using 2> or 2>>.
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 +
But sometimes, you need stderr to be included in stdout for certain purpose, e.g. when another program is parsing/logging only stdout and you need errors to be in there, too. In these cases, you can redirect stderr to stdout with "2>&1". (Of course, it is also possible to redirect stdout to stderr this way, if you need to silence a program on stdout for some purpose).
 +
 +
Examples:
 +
cat notexisting 2> test  # Writes "File or directory not found." to test
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cat notexisting 2>&1  # Outputs "File or directory not found." to stdout
 +
 +
= | =
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| is used to pipe the stdout of one programm to the stdin of another one.
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tail -n 100 ''foo'' | sort -r  # Reverse sort the last 100 lines of the file ''foo''
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 +
= && =
 +
&& evaluates the binary result of the command left of it and ands it with the binary result right of it if it is not already false, where false!=0. In other words, if you write
 +
command1 && command2
 +
 +
command2 will only be executed if command1 returned success.
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 +
= See also =
 +
* [[bash operator]]
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* [[shell Programming]]

Latest revision as of 13:22, 21 March 2021

You have a shell script and wonder what these operators do? Then this article is for you.

* and ?

You may already know these used for so called "shell globbing" (pattern matching). * replaces an arbitrary amount of characters (including none), while ? replaces exactly one character.

These wildcards are actually bash operators! Imagine, we have the files a1, a2:

cp a*

now is a valid bash command which overrides one of these files with the other one.

$()

The operator $() in the bash shell is replaced by the output of the command enclosed in the parentheses. It is equivalent to backticks (``), but can be cascaded more easily.

Examples

rpm -ql $(rpm -qa)
for i in $(seq 1 1 100); do echo $i; done

$(())

The operator $(()) in the bash shell is replaced by the arithmetic result of the expression enclosed in the parentheses.

Examples:

# echo $((2*2))
4

$$

The operator $$ delivers the id of the currently-running process.

Example:

# kill -9 $$

kills the current process

$!

The operator $! delivers the id of the process that has most recently sent to the background.

Example:

# xosview & sleep 5; kill $!

shows xosview for 5 seconds and stops it

> and >>

> and >> can be used to redirect the output (only stdout, not stderr, for that see below) of a command to a file. The difference between them is, that >> appends to the given files, while > will truncate it.

Examples:

ls -alh > files-in-directory.txt
ls -alh /otherdir >> files-in-directory.txt  # Append list of files in other directory

2> or 2>>

stderr can be redirected by using 2> or 2>>.

But sometimes, you need stderr to be included in stdout for certain purpose, e.g. when another program is parsing/logging only stdout and you need errors to be in there, too. In these cases, you can redirect stderr to stdout with "2>&1". (Of course, it is also possible to redirect stdout to stderr this way, if you need to silence a program on stdout for some purpose).

Examples:

cat notexisting 2> test  # Writes "File or directory not found." to test
cat notexisting 2>&1  # Outputs "File or directory not found." to stdout

|

| is used to pipe the stdout of one programm to the stdin of another one.

tail -n 100 foo | sort -r  # Reverse sort the last 100 lines of the file foo

&&

&& evaluates the binary result of the command left of it and ands it with the binary result right of it if it is not already false, where false!=0. In other words, if you write

command1 && command2

command2 will only be executed if command1 returned success.

See also